The present invention relates to an off-road vehicle, such as a tractor, having a front axle with compound steering and a power takeoff drive coupling mounted for movement with the steerable axle, wherein the engine has a power takeoff shaft connected to the drive coupling by a transmission train, the axis of the engine power takeoff shaft being vertically offset from the axis of the drive coupling.
U.S. Pat. No. 4,825,970 describes an adaptation of a conventional road vehicle, such as a car, van or a truck to allow the engine to drive a power takeoff (PTO) coupling. A propeller shaft leading to the PTO drive coupling extends to a point below the crankshaft pulley and is driven by means of a chain that passes over a sprocket mounted on the crankshaft and a sprocket mounted on the propeller shaft. In this patent specification, the PTO coupling cannot move significantly in relation to the engine and it is not even necessary to provide universal joints in the propeller shaft. The transmission train connecting the engine to the PTO drive coupling in this patent is therefore totally unsuitable for a vehicle in which the PTO drive coupling is mounted for movement with a steerable axle, as present in some tractors or other off-road vehicles fitted with a compound steering system.
Compound steering is used in off-road vehicles to reduce the turning circle. In addition to the wheels pivoting about a vertical axis relative to the front axle, the axle as a whole pivots about a vertical axis relative to the vehicle centreline. The pivot axis of the axle is often set back from the axis of rotation of the wheels so that the axle swings from side to side at the same time as pivoting. This allows the steerable wheels to be turned to a greater extent without colliding with the engine or chassis of the tractor.
A front PTO drive coupling that is mounted for movement with the steerable axle of an off-road vehicle having compound steering is known from EP-A-0.691.255. Here, the PTO drive coupling is arranged below and forwardly of a PTO drive shaft that projects from the front end of the engine. The PTO drive coupling is connected to the engine PTO drive shaft by a transmission train that consists of an inclined telescopically extendible propeller shaft having universal joints at both ends.
A problem with this known design is that torque is never transmitted in a straight line from the engine power takeoff shaft to the drive coupling. Even when the vehicle is being driven forwards in a straight line, the drive coupling and the engine power takeoff shaft do not lie in line with one another. Instead, the propeller shaft is inclined in a vertical plane relative to the engine power takeoff shaft and the power takeoff drive coupling and power is transmitted at an angle through the universal joints at all times. When the vehicle is steered, the angle through which torque needs to be transmitted is increased still further. The maximum angle through which the universal joints of the connecting shaft must be capable of transmitting torque is therefore even greater than the steering angle of the axle and this large angle places great demands on the universal joints, adding to their expense and increasing the wear to which they are subjected.
The present invention therefore seeks to provide a vehicle having a PTO drive coupling mounted on a steerable axle in which the foregoing disadvantages are mitigated.